The PID-regulator is very common for the control of industrial processes and provides for proportional, integrating and derivative control. A process of larger scope employs a large number of such regulators. PID-regulators are manufactured in large series as standard products. It is more and more common that the regulators are based on microcomputers, and then more complicated control functions can be used.
Even if the regulator is based on a microcomputer the principal structure of a conventional PID-regulator is maintained since persons in the industry skilled in the art have a long and experienced knowledge about and a feeling for the tuning of such PID-regulators.
There are well established methods, e.g. the method of Ziegler and Nichols, for the manual tuning of a PID-regulator in dependence upon the parameters of the process. In spite of this many regulators in industrial processes are badly tuned in practise. This is due on one hand to the fact that the manual tuning which comprises manually changing the regulator amplification is tedious, on the other hand to the fact that the parameters/properties of the process change over the course of time.
There is also equipment for automatic tuning of PID-regulators but such equipment is expensive and not quite simple to use. Moreover, there are adaptive regulators but such regulators are much more complicated than a simple PID-regulator and have not yet been used on a large scale.
Thus, there is a need for a simple method of automatic tuning of a PID-regulator which method results in a non-expensive regulator. The method should be so simple that it can be applied on PID-regulators realized by means of a microcomputer only by making a simple change of, or a minor addition to the program of the regulator.
The purpose of the invention is to facilitate a simple method of tuning a PID-regulator and as a step thereof to provide a method and an apparatus for bringing the system including the PID-regulator into controlled self oscillation. When the system oscillates, quantities of the process which are essential for the tuning can be measured.
This purpose is achieved by means of a method where the process and the regulator in common have a transfer function G(s) in a feed-back system and the system is brought into controlled self oscillation for measuring the amplitude and the frequency of said oscillation whereupon the regulator is tuned in dependence upon the values measured for the amplitude and the frequency of said oscillation. In accordance with the invention the signal fed to the regulator is subjected to the effect of a circuit function (NL) having a non-linear characteristic and having a describing function N(A). A relation G(i.omega.).multidot.N(A)=-1 is valid for at least one value of the angular frequency and the amplitude A of said signal .
The method facilitates simple automation of the tuning of PID-regulators, particularly regulators based on a microcomputer.